DNA-topoisomerases are enzymes present in the nuclei of cells where they catalyze the breaking and rejoining of DNA strands, controlling the topological state of DNA. Recent studies also suggest that topoisomerases are involved in regulating template supercoiling during RNA transcription. There are two major classes of mammalian topoisomerases. DNA-topoisomerase-I catalyzes changes in the topological state of duplex DNA by performing transient single-strand breakage-union cycles. In contrast, mammalian topoisomerase II alters the topology of DNA by causing a transient enzyme bridged double-strand break, followed by strand passing and resealing. Mammalian topoisomerase II has been further classified as Type II xcex1 and Type II xcex2. The antitumor activity associated with agents which are topoisomerase poisons is associated with their ability to stabilize the enzyme-DNA cleavable complex. This drug-induced stabilization of the enzyme-DNA cleavable complex effectively converts the enzyme into a cellular poison.
Several antitumor agents in clinical use have potent activity as mammalian topoisomerase II poisons. These include adriamycin, actinomycin D, daunomycin, VP-16, and VM-26 (teniposide or epipodophyllotoxin).
In contrast to the number of clinical and experimental drugs which act as topoisomerase II poisons, there are currently only a limited number of agents which have been identified as topoisomerase I poisons. Camptothecin and its structurally-related analogs are among the most extensively studied topoisomerase I poisons. Recently, bi- and terbenzimidazoles (Chen et al., Cancer Res. 1993, 53, 1332-1335; Sun et al., J. Med. Chem. 1995, 38, 3638-3644; Kim et al., J. Med. Chem. 1996, 39, 992-998), certain benzo[c]phenanthridine and protoberberine alkaloids and their synthetic analogs (Makhey et al., Med. Chem. Res. 1995, 5, 1-12; Janin et al., J. Med. Chem 1975, 18, 708-713; Makhey et al., Bioorg. and Med. Chem. 1996, 4, 781-791), as well as the fungal metabolites, bulgarein (Fujii et al., J. Biol. Chem. 1993, 268, 13160-13165) and saintopin (Yamashita et al., Biochemistry 1991, 30, 5838-5845) and indolocarbazoles (Yamashita et al., Biochemistry 1992, 31, 12069-12075) have been identified as topoisomerase I poisons.
Presently, a need exists for novel anti-cancer agents, for anti-cancer agents that exhibit improved activity, and for anti-cancer agents that exhibit fewer side-effects or improved selectivity compared to existing agents.
The present invention provides compounds that exhibit inhibitory activity against topoisomerase I, and compounds that are effective cytotoxic agents against cancer cells, including drug-resistant cancer cells. Accordingly there is provided a compound of the invention which is a compound of formula I: 
wherein
R1 and R2 are each independently hydrogen, (C1-C6)alkyl, (C3-C6)cycloalkyl, (C1-C6)alkoxy, nitro, hydroxy, halo(C1-C6)alkyl, trifluoromethoxy, halo, (C3-C6)cycloalkyl(C1-C6)alkyl, (C1-C6)alkanoyl, hydroxy(C1-C6)alkyl, (C1-C6)alkoxycarbonyl, (C1-C6)alkylthio, (C2-C6)alkanoyloxy, aryl or heteroaryl; or R1 and R2 taken together are methylenedioxy; or R1 and R2 taken together are benzo; wherein any aryl, heteroaryl, or benzo may optionally be substituted by 1, 2, or 3 substituents independently selected from the group consisting of (C1-C6)alkyl, (C3-C6)cycloalkyl, (C1-C6)alkoxy, nitro, hydroxy, halo(C1-C6)alkyl, trifluoromethoxy, (C3-C6)cycloalkyl(C1-C6)alkyl, (C1-C6)alkanoyl, hydroxy(C1-C6)alkyl, (C1-C6)alkoxycarbonyl, (C1-C6)alkylthio, (C2-C6)alkanoyloxy, and halo;
R3 is hydrogen, (C1-C6)alkyl, (C3-C6)cycloalkyl, (C1-C6)alkoxy, nitro, hydroxy, halo(C1-C6)alkyl, trifluoromethoxy, (C3-C6)cycloalkyl(C1-C6)alkyl, (C1-C6)alkanoyl, hydroxy(C1-C6)alkyl, (C1-C6)alkoxycarbonyl, (C1-C6)alkylthio, (C2-C6)alkanoyloxy, or halo; and
R4 and R5 taken together are a 3, 4, or 5 membered saturated or unsaturated chain comprising members selected from the group consisting of non-peroxide oxygen, sulfur, N(X), and carbon, optionally substituted by oxo; wherein each X is independently absent or is H, O (C1-C4)alkyl, phenyl or benzyl; and wherein at least one (e.g. 1 or 2) of said chain members is an Nxe2x80x94H group;
or a pharmaceutically acceptable salt thereof,
provided R4 and R5 taken together are not xe2x80x94N(H)xe2x80x94C(H)xe2x95x90Nxe2x80x94.
Preferrably, any carbon of R4 and R5 is saturated (xe2x80x94CH2xe2x80x94) or unsaturated (xe2x95x90CHxe2x80x94).
The invention also provides a pharmaceutical composition comprising a compound of formula I, or a pharmaceutically acceptable salt thereof, in combination with a pharmaceutically acceptable diluent or carrier.
The invention also provides a therapeutic method comprising inhibiting cancer cells by administering to a mammal (e.g. a human) in need of such therapy, an amount of a compound of formula I, or a pharmaceutically acceptable salt thereof, effective to inhibit said cancer cells.
The invention also provides a method comprising inhibiting cancer cells by contacting said cancer cells in vitro or in vivo with an amount of a compound of formula I, or a pharmaceutically acceptable salt thereof, effective to inhibit said cancer cells, i.e. to inhibit their activity, such as their ability to divide, migrate, or proliferate.
The invention also provides a compound of formula I for use in medical therapy (preferably for use in treating cancer, e.g. solid tumors), as well as the use of a compound of formula I for the manufacture of a medicament useful for the treatment of cancer, e.g. solid tumors.
The invention also provides processes and novel intermediates disclosed herein which are useful for preparing compounds of the invention. Some of the compounds of formula I are useful to prepare other compounds of formula I.